The present invention relates to ultraviolet (xe2x80x9cUVxe2x80x9d) reflectors and UV-based light sources incorporating the same. More particularly, the present invention relates to visible light sources that are based on semiconductor UV light-emitting devices and have reduced UV emission to the surroundings.
Semiconductor light-emitting diodes and laser diodes (hereinafter collectively called xe2x80x9cLEDsxe2x80x9d) have gained importance as light sources in many applications because of their low power consumption with relatively good luminescence efficiency and of recent advances in manufacturing technology of GaN-based semiconductor devices. Light sources have been disclosed in which radiation or light from UV light- or blue light-emitting LEDs is converted to light having generally longer wavelengths, particularly in the visible range. In the present invention disclosure, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9clightxe2x80x9d are used interchangeably to mean electromagnetic radiation having wavelengths in the range from UV to infrared; i.e, from about 100 nm to about 1 mm.
One common method for generating visible light from LEDs is to dispose a phosphor composition adjacent to the LED to convert radiation emitted by the LED to visible light. LED-based lighting devices that use phosphors in a dispersed layer around the LED for light conversion often suffer from the undesirable halo effect and the penumbra effect. The halo effect occurs because of the poor mixing of light emitted by the phosphor and that by the LED. The LED generally emits light of one wavelength range; i.e., of one color, in a directional, anisotropic fashion. However, the dispersed phosphor emits light of another wavelength range; i.e., of a different color, isotropically (i.e., in all directions). Therefore, the light output from the system appears to have different colors when it is viewed at different angles. When light from such a light source is directed onto a flat surface, it appears as a halo of one color surrounding another color. The penumbra effect is similar to the halo effect, except that the halo effect is an effect of poor color mixing, while the penumbra effect is an effect of non-uniform light intensity. The penumbra effect causes the LED-based light source to appear brighter at the center than at the edges. As noted above, the LED emission is directional, while the phosphor emission is isotropic. Therefore, the overall light emitted by the LED-based light source appears brighter at the center because the LED chip emission intensity is greatest in this region. In order to remedy these problems, particles of a color diffuser can be added into the phosphor layer to mix the colors emitted by the LED and the phosphor. For example, U.S. Pat. No. 6,066,861 briefly discloses the use of CaF2 as a diffuser for xe2x80x9coptimizing the luminous pattern of the component.xe2x80x9d Similarly, U.S. Pat. No. 6,069,440 mentions the use of a xe2x80x9cdispersantxe2x80x9d; such as barium titanate, titanium oxide, aluminum oxide, and silicon dioxide; together with the phosphor for xe2x80x9cblending colors.xe2x80x9d However, these patents do not disclose or suggest that dispersants are used in these or similar devices for any benefits other than color mixing or blending or what the desired characteristics of these dispersants are.
UV light-emitting LEDs are particularly preferred in applications in which visible light is extracted because the color of the light emitted by UV LED-based lamps is controlled largely by the phosphor blend since the UV LED chip does not contribute significantly to the visible light emission. As used herein, the term xe2x80x9cUV LEDsxe2x80x9d means LEDs that emit UV radiation having wavelengths less than or equal to about 420 nm. However, as the wavelengths of radiation emitted by the LED becomes shorter, and the radiation, thus, becomes more energetic, there is an increasing need to ensure that UV radiation preferably does not escape substantially from the lighting device into the surrounding environment.
U.S. Pat. Nos. 5,813,752 and 5,813,753 disclose a UV/blue LED-phosphor device that emits visible light. In U.S. Pat. No. 5,813,752 the UV/blue LED is disposed on a sapphire substrate and a phosphor layer is applied directly on the UV/blue LED. A long-wave pass (xe2x80x9cLWPxe2x80x9d) filter preferably composed of a multilayer dielectric stack of alternating materials having high and low refractive indices is disposed directly on the phosphor layer. In U.S. Pat. No. 5,813,753 the UV/blue LED is disposed in a cup. In both patents, the UV/blue LED emits radiation in the UV-to-blue wavelength range. The cup is filled with an epoxy having a UV/blue-excitable phosphor dispersed therein that converts part of the UV/blue radiation to visible light. A LWP filter, preferably composed of alternating layers of dielectric materials having high and low refractive indices, is disposed on top of the phosphor layer. The LWP filter is believed to reflect UV/blue light back to the phosphor and to transmit visible light emitted by the phosphor. However, these patents do not teach the selection of the materials having high and low refractive indices, the design, or the construction of the multilayer LWP filter to achieve these goals. Material selection is among the critical considerations for the success of such a filter in a device of this nature because the effectiveness of the filter in reflecting UV radiation depends, among other things, on the refractive index of the layer disposed adjacent to the epoxy layer relative to the refractive index of the epoxy.
Therefore, there still is a need to provide improved UV-radiation reducing filters and improved UV LED-based lighting devices that allow only a minimum amount of UV radiation leakage to the surrounding environment. In addition, it is also very desirable to provide a light source that has uniform color and light intensity and at the same time low UV radiation leakage.
The present invention provides UV reflectors comprising materials that scatter or reflect UV radiation having wavelengths less than or equal to about 420 nm so that its transmission in the forward direction is reduced compared to the transmission of light having other wavelengths, especially light having wavelengths in the visible spectrum. Generally, a UV reflector of the present invention is made of a composite material of at least two materials having different refractive indices. In a first embodiment, the UV reflector is a composite structure of a first material in a particulate form substantially dispersed in a second solid material that is substantially transparent with respect to light of the visible spectrum. In a second embodiment, the UV reflector is a layered structure of materials having at least two different refractive indices. The refractive index of a first material is less than or equal to and that of a second material is greater than the refractive index of the medium through which light travels before impinging on the UV reflector.
The present invention also provides UV-based light sources having a uniform color and light intensity and reduced UV radiation leakage. The UV-based light source comprises a LED emitting UV radiation, a shaped structure that comprises a molding or casting material covering the LED, particles of at least one phosphor that is excitable by the UV radiation emitted by the LED and particles of at least one UV-radiation scattering material dispersed substantially uniformly in at least a portion of the molding or casting material. The particles of the phosphor composition and the scattering material are disposed in the vicinity of the LED. The molding or casting material may be a glass or a polymeric material that is substantially transparent after curing. As used herein, a substantially transparent material is defined as one that transmits greater than 80% of incident light having a wavelength of 555 nm at an incident angle of less than 10 degrees.
In another aspect of the present invention, the UV-radiation scattering material has a refractive index substantially different from that of the molding or casting material. The refractive index or index of refraction of a material, as referred to herein, is that measured for light having the wavelength of 555 nm.
In another aspect of the present invention, the UV-radiation scattering material is a dielectric material of which greater than 95% of a representative population of particles has particle diameters less than about half of the maximum wavelength of UV radiation in the molding or casting material and which has a mean particle diameter less than about one-tenth of the same wavelength. The diameter of a particle having irregular shape is defined herein as the diameter of a sphere equaling the largest dimension of the particle. The mean particle diameter is the average particle diameter of a representative sample of the particles.
In another aspect of the invention, the UV-based light source further comprises a UV reflector that comprises a plurality of layers of materials having at least alternating first and second refractive indices and that is disposed on the shaped structure of a molding or casting material. The first refractive index is greater than about 1.5, and the second refractive index is less than about 2. Each of the layers has a thickness of one-quarter or an even multiple of one-quarter of the wavelength of the radiation to be transmitted.
In still another aspect of the present invention, the surface of the UV reflector adjacent to the molding or casting material has a plurality of protrusions that have a typical dimension of much less than the wavelength of UV radiation in the molding material.
Other features and advantages of the present invention will be apparent from a perusal of the following detailed description of the invention and the accompanying drawings in which the same numerals refer to like elements.